Looking for knobs for 900 Receiver.

Does anyone know where I might be able to locate one of the blade shaped knobs that go on the Monitor or Dubbing controls for a 900 Receiver? The one I had on the Monitor control cracked into three pieces on me and the other one has a hairline crack in it (but is still holding on). I bought this new back in the mid 80's and hate to see it looking so forlorn.

Thanks